This invention relates to a discharge lamp lighting device employing a booster circuit for starting and/or reigniting discharge lamp means. More particularly the invention relates to oscillation circuit arrangements used in the lighting device for improving the characteristics of the discharge lamp or lamps.
In recent years various types of discharge lamp lighting devices have been developed for using energy in an optimal manner to save energy resources. In such devices an oscillation circuit or booster is used for starting and/or reigniting a discharge lamp, as disclosed in U.S. Pat. Nos. 3,665,243; 3,753,037; 3,866,088; 3,942,069; and 4,145,638 for the starter, and U.S. Pat. No. 4,079,292; and a copending U.S. Patent Application Ser. No.: 873,241, filed on Jan. 30, 1978, now U.S. Pat. No. 4,238,708, issued Dec. 9, 1980, for a reigniting device. The following explanations shall facilitate the understanding of conventional lighting devices and the function and problems of electronic starting systems and of the so-called "every half cycle ignited system". A conventional lighting device for discharge lamps is first explained by referring to the circuit of FIG. 1, comprising an a.c. power source 1, a current limiting ballast choke 2, a discharge lamp 3 with filaments 4, 5 connected in series with the source 1 through the choke 2, and an oscillation circuit 6 for generating a high frequency and high voltage output. The oscillation circuit 6 connected to the lamp 3, comprises an oscillation capacitor 7 connected between filaments 4 and 5 at the source side, and a series circuit formed by a voltage step-up or nonlinear inductor 8 and a thyristor 9, which is connected between filaments 4 and 5 at the side opposite the source. In the operation, the oscillation capacitor 7 is charged by supplying an a.c. power from the source 1. The thyristor 9 becomes conductive when the terminal voltage of the capacitor 7 exceeds the break-over voltage of the thyristor 9. Thus, a high frequency oscillating voltage is generated by the cooperation of the capacitor 7 and the inductor 8. This output voltage Vo is higher than the source voltage "e" and is applied to the lamp 3. While the low frequency current of the a.c. power source 1 flowing through the thyristor 9 gradually increases, until the current exceeds a holding current thereof, the thyristor 9 maintains its continuous conductive state so as to stop the oscillation . Then, due to the repetition of the above operations in each half cycle, the oscillation circuit 6 repeats the high frequency oscillation. Meanwhile, the filaments 4, 5 of the lamp 3 are preheated by the overlapped current of the oscillating current during the oscillation period of the oscillation circuit 6, and the input current flows in a closed circuit of elements 1-2-4-8-9-5-1, when the thyristor 9 is in the conductive state. Thus, as the high frequency oscillation continues to preheat the filaments 4, 5 to a sufficiently preheated state, the discharge lamp 3 starts its initial ignition by the output voltage Vo of the oscillation circuit 4. When the lamp 3 is operated at the initial ignition, the thyristor 9 becomes nonconductive and the operation of the oscillation circuit 6 ceases.
FIG. 2 illustrates a more recent conventional lighting device for discharge lamps, wherein an intermittent oscillation circuit 10 is used to operate a discharge lamp 3 for its initial ignition during a starting period and its reignition at each half cycle of the a.c. power source 1 in the normal lighting operation. This circuit is very efficient in its use of energy and has achieved a ballast choke of minimal size and weight, thereby also saving energy.
Although similar to the circuit of FIG. 1, the lighting device of FIG. 2 comprises the a.c. power source 1, a ballast choke 2, a discharge lamp 3, the oscillation circuit 6 and further a second capacitor 11 for an intermittent oscillation in series connection with the oscillation circuit 6 for generating a high frequency and high voltage oscillation. That is, the intermittent oscillation circuit 10 comprises a series circuit of the second capacitor 11 for an intermittent oscillation and the oscillation circuit 6 including the oscillation capacitor 7, nonlinear inductor 8 and thyristor 9. As far as the generation of a high frequency and high voltage output is concerned, the intermittent oscillation circuit may be replaced by another type of booster circuit using a gated thyristor such as a triac or an inverter.
FIG. 3 shows a set of operational voltage wave forms for the lighting device of FIG. 2, which may be calculated with the aid of an equivalent circuit of FIG. 2, but high frequency components are omitted from each of the wave forms of FIG. 3, excepting FIG. 3(D). By switching on the power source, the source voltage "e" in FIG. 3(A) is supplied to the lamp 3 through the choke 2 and the intermittent oscillation circuit 10, and also to the thyristor 9 through the second capacitor 11. The thyristor 9 becomes conductive when the source voltage "e" rises to the break-over voltage thereof, and the oscillation circuit 6 generates an output in cooperation of the first oscillation capacitor 7 and the inductor 8. The oscillating operation will be continued if the second intermittent oscillation capacitor 11 is not inserted, but due to the oscillating action of the oscillation circuit 6 the second intermittent oscillation capacitor 11 is gradually charged until the terminal voltage of the first capacitor 7 cancels the source voltage "e" and the oscillation circuit 6 starts to oscillate intermittently at an initial time period of each half cycle of the source voltage "e". Accordingly, the intermittently oscillating circuit 11 generates an intermittent oscillation output V.sub.R at a fixed phase of each half cycle of the a.c. source voltage "e".
The intermittent oscillating output V.sub.R is supplied to the discharge lamp 3 together with the source voltage "e" as shown in FIG. 3(D). At the same time, an input current i.sub.R in the circuit 6 flows through a circuit path of the power source 1, ballast choke 2, filament 4, intermittent oscillation circuit 10, filament 5, and back to the power source 1. Accordingly, the filaments 4, 5 of the discharge lamp 3 are preheated by the current i.sub.R. Thus, preheated filaments 4, 5 lower the initial starting voltage of the discharge lamp 3 which is lit by the sum of the source voltage "e" and the oscillating output V.sub.R. After the lamp 3 is lit, the lamp current i.sub.T of the discharge lamp 3 flows through the ballast choke 2 as shown in FIG. 3(C). Also, since the choke impedance is changed, the occurring period of the input current i.sub.R becomes shorter than that of the preheating stage. Actually, during the suspended or ceased time of the input current i.sub.R, the oscillation circuit 6 stops its oscillation, and accordingly the preheating of the filament by the input current i.sub.R decreases while the lamp 3 sustains its lit condition at each half cycle. Also, during the suspended time of the input current i.sub.R, preheating of the filaments 4, 5 ceases. After the initial lighting of the lamp 3 is started, the lamp maintains its burning state by reignition due to the oscillation output V.sub.R of the intermittent oscillation circuit 10 at each half cycle of the power source 1.
Here, a lamp voltage V.sub.T shows a rectangular wave form with a portion corresponding to the suspended time in the intermittent oscillating period as shown in FIG. 3(B), and its effective value V.sub.T shows a rather lower value compared with that of conventional lighting systems. Further, as shown in FIG. 3(E), due to the flow of the intermittent input current i.sub.R through the ballast choke 2, the wave form of the lamp voltage V.sub.T somewhat rises under the influence of the input current i.sub.R. An appearing phase of the input current i.sub.R is almost constant regardless of any variation of fluctuation of the source voltage, and accordingly the initial phase of the lamp current i.sub.T is maintained to be an almost constant phase regardless of any fluctuation of the source voltage "e". Also, the input current i.sub.R has a negative coefficient characteristic to decrease if the lamp current i.sub.T increases as the source voltage rises due to an encroachment of a remaining portion of the wave of the lamp current i.sub.T upon the occurring period of the next half cycle input current i.sub.R. For this reason the fluctuation rate of the lamp current i.sub.T in the "each half cycle ignited lighting system" is preferably maintained regardless of any reduction of a stabilizing impedance.
FIGS. 3(F) and 3(G) show wave forms of the instantaneous reactive power (V.sub.CH.i) and accumulated energy S of the ballast choke 2 which are calculated from the lamp voltage V.sub.T, lamp current i.sub.T, input current i.sub.R of the intermittent oscillation circuit 10, oscillating output voltage V.sub.R, and source voltage "e". Namely, in FIG. 3(F), S1 is the energy accumulated by the input current i.sub.R in the oscillating period between t1 and t2, S2 is the energy accumulated by the lamp current i.sub.T in the period between t2 and t3 during which the source voltage "e" exceeds the lamp voltage V.sub.T, S3 is the energy released by the lamp current i.sub.T in the period between t.sub.3 and t.sub.4 during which the lamp voltage V.sub.T exceeds the source voltage "e", and the expression S1+S2=S3 applies.
The accumulated energy and the necessary inductance of the ballast choke 2 are calculated from the wave forms as shown in FIG. 2 and indicate respectively to be about a quarter and a fifth compared with those of conventional glow start systems, accordingly the "each half cycle ignited system" is capable of minimizing the ballast choke 2 in accordance with the above ratio.
Further, in comparison with a ballast of the rapid start system providing a step-up transformer, the minimizing ratio becomes even more remarkable. Moreover, according to said lighting system, the phase difference between the source voltage "e" and the lamp current i.sub.T is smaller than that of the conventional lighting systems, therefore, it is possible to omit a power-factor improving capacitor or to use a capacitor having an extremely small capacity.
As described above, the "every half cycle lighting system" has a remarkable advantage of saving resources and energy in comparison with the conventional lighting systems, it also has an excellent fluctuation of the lamp current, and permits the miniaturization of the ballast in comparison with that of the conventional lighting systems. FIG. 4 is a characteristic diagram showing the necessary starting voltage Vst and oscillating voltage Vo or V.sub.R as a function of time t during the starting period, when the discharge lamp 3 is operated by the conventional lighting devices shown in FIGS. 1 and 2 respectively. Referring to FIG. 4 when the filaments of the discharge lamp 3 are in the "cold cathode" state, the filaments 4, 5 are not sufficiently preheated and a comparatively high initial ignition voltage or starting voltage Vst is required to initiate the operation of the discharge lamp 3. However, when the filaments 4, 5 are sufficiently preheated, namely, when they are in the "hot cathode" state, the lamp 3 starts its ignition even with a comparatively low starting voltage. Further, each of the filaments 4, 5 has a characteristic such that the resistance value is low at the "cold cathode" state, but it becomes high at the "hot cathode" state. Also, if filaments are included in the oscillation circuit 6 as shown in FIG. 1, the oscillating output voltage is lowered by the resistance components of the "hot cathode" state filament. Therefore, it is necessary to select a capacitance value for the oscillation capacitor and an inductance value for the inductor to provide a counterbalanced oscillating output voltage. However, if these values in the circuits of FIGS. 1 and 2 are selected accordingly, an extremely high oscillating output voltage Vo or V.sub.R is caused when the filaments 4, 5 are in a "cold cathode" state, and if such high oscillating voltage Vo or V.sub.R above the starting voltage Vst is supplied to both filaments in a "cold cathode" state, defects caused by sputtering due to a cold cathode glow discharge and a short life of the discharge lamp 3 will occur. Accordingly, as shown by dotted lines V.sub.D1 or V.sub.D2, it is desired ideally to provide a characteristic curve for the output voltage, so that the oscillating voltage V.sub.D1 or V.sub.D2 is restricted for a given period of initial time after the power source voltage is supplied. In other words, after the filaments 4, 5 are sufficiently preheated it is desired that the oscillating voltage rises to start the discharge lamp 3 in its lighting operation.